Mk 20 Mod 0 grenade launcher

The Mk 20 Mod 0 was a 40mm automatic grenade launcher manufactured by NOS Louisville which was used by the US Navy during the Vietnam War. The Mk 20 was eventually replaced by the Mk 19 Mod 3.

Design
It used a blow-forward mechanism that was previously used on semi-automatic pistols such as the Steyr Mannlicher M1894 and Schwarzlose Model 1908.